Induction motor

English-speaking guide called asynchronous induction motor. Immediately stand point the i. Internet is hammered with questions of differences of this type of machinery, the nuances of the collector, synchronous engines, in fact, comes easy. The only type of engine that generates a phenomenon of induction poles. Other designs use permanent magnets, coils supplied at... Just in induction (asynchronous) engines used pickup, creating a driving force. Factor determines the feature - the rotational speed difference of the field frequency.

induction motor apparatus

The stator of an induction motor

To begin the simplest common goal: AC power supplied to the stator windings. Look at the photo: a typical example of the stator. Removing the rotor, it is impossible to say which type of engine belongs core twined copper. We got the main conclusion: the stator does not specify the method of forming the driving force. Rather, the support protrudes with respect to which the stator acts.

We see composite core comprising two coils. winding direction creates two distinct poles. It can not be called thickening field north or south, since the direction of the lines is constantly changing (at twice the frequency of 100 Hz networks). The assembly is carried out as follows:

1. Coils harassed separately. Designers know how many turns you need any wire lead.
2. The obtained hank put gently on magnetic spacers (traditional form letter T). For insulation layer paving vinyl another polymer.
3. Then winding the ends slightly bends to the periphery, the turns tightly abut the base of the letter T.
4. In this case, the core component, the inner part of the coils is inserted into the outer ring. But more simpler design.

The core is assembled from plates isolated from each other by means of lacquer. The work of the induction motor 230 volts alternating field induces eddy currents, causing magnetization reversal effect. To reduce losses, the core is divided into plates. Stainless steel, alloy additions of silicon provides a low electrical conductivity.

The domestic induction motors stator two poles. There are exceptions to the rule. On the other picture we see the stator asynchronous outdoor fan motor with three speeds. Eight poles to power up a pile of iron, needed a condenser. It shifts the phase of the voltage at minus 90 degrees with respect to current. It becomes possible to create a variable rotating field in the stator. This type of induction motor is called a capacitor.

The first two phases of use offered Nikola Tesla.

The scheme is as follows:

1. Four winding lying in the cross peaks are powered network of 230 volts. Two - opposing - have the same sign pole, the other - the other. So, the field rotates at half the speed of the network (25 Hz). This is enough serviceable fan operation.
2. Soft start of the induction motor and the operation is possible only in conditions when the field is flattened. For this purpose, four winding applied lying diagonally. Here, the voltage is shifted by 90 degrees. Using auxiliary coils technical characteristics are improved.

How to adjust speed? Regulators induction motor speed commute winding. control keypad is arranged at any one time to allow pressing of a button, or any. Eight coils have a couple of taps. Stator made necessary switching, some branches are powered condenser. Pressing each button activates the work of the winding. Fully stator runs at full speed.

An exemplary diagram showing the operating principle, illustrated by photos. The rotation speed is set switched windings buttons 1, 2, 3. The need to protect against simultaneous switching is dictated by the requirements for normal operation. The result is a simplest method to control the speed.

Magnetic core composed of electrical steel sheets, which reduces losses in heating. The temperature may be significant, however asynchronous fan motor provided with rotor blades (see. Photo). Any fan can really only heat the air, not vice versa.

rotor induction motor

In this case, the engine will provide long-term operation. Therefore, the rotor is provided with a tangential fan blades. It helps cool design hot summer nights. The owner can sleep, ignoring the possibility of fire. Any good device operates in a similar manner (itself off). In this case, the engine is constructed according to the scheme with short-circuited rotor. On the shaft of the drum sits where in silumin recessed copper wires. Short-circuited to each other annular connector. Such a solution in the literature usually called squirrel cage (wheel) for obvious reasons.

Asynchronous squirrel-cage motor is dominant in everyday life. Field induced in the stator conductors, then going through the clutch ether, the shaft grows. Never catch up with the network frequency. Because induced currents are zero, the clutch is broken. Shaft brakes, again picked up the field. Similarly operate single-phase asynchronous motors, any other. In essence, there is no difference, whereby an alternating field.

Assign one or more large family. induction motor device is fundamentally different. The rotor is provided with windings as a collector motor. Typically, three-phase. This will allow to bring a much stronger field, there is a major problem: difficult to budge the shaft. The enormous field strength constitutes incredible strength adhesion, whereby it is possible equipment failure. Also shaft generally do not spin up.

That is why to reduce the strength of the induced currents (field strength) in the circuit of rotor phase cut one rheostat. Active resistance prevents EMF develop shaft power: a certain amount of heat dissipated by the Joule formed by active resistance. Starting torque of the asynchronous motor with slip-ring motors large enough speed breakdown does not occur. It is clear that the value of the rheostat resistance for each design their own. Determine the number of the induction motor rotor, predetermined characteristics, the starting load.

Note that in all cases with asynchronous motors are seeing big losses. A particularly well illustrated by a rheostat. Power induction motor is directly spent on the heat dissipated. The main advantage of this class is still considered the simplicity of construction and maintenance equipment. Otherwise any type of induction motors have been thrown into the dustbin of history.

How does an induction motor

The stator generates a rotating magnetic field. The direction of the tension lines is determined by the rule of thumb (right hand). Therefore, a stator until set aside, we try to understand what is happening in parallel to the rotor. Let's start squirrel cage.

Inside the stator there is a field, whose lines of tension in the first approximation, directed toward the center where the shaft. Squirrel cage conductor intersect at an angle close to 90 degrees. According to the right-hand rule alternating field induces an electromotive force which generates current. As a result, there is an answer.

Any pair of squirrel cage conductors is drawn into the frame. It revolves around the stator field. By the rule of arms there is a response field which is directed opposite to the original:

1. Rotor stator moves slower. Let the rotation describes the hour hand.
2. At some point, the North Pole begins to catch up with one of the conductors of the cage.
3. The current is directed so that the circular line of tension response magnetic fields going toward the pole.
4. So, ahead of you pole encounters the eponymous sign of the charge, it starts to push it. Behind the image of the "south", tries to run after the field.

A simple short explanation of why the squirrel cage, in the end, starts to rotate. The rotor should not be too heavy, golf grip is not very strong. This explains the low tractive force developed by an induction motor at the start. Inrush current is high, because nothing prevents the generation of fields within the stator. Note: the rotor single-phase asynchronous motor, shown in the photo at the beginning of the article, guides the cage a little inclined to the axis of the drum. It helps to create a more uniform magnetic pole by compensating for disadvantages (primarily non-uniformity) of the stator field rotation.

Phase rotor winding consists of, the normal of which is directed approximately at the center of the motor (shaft). You can imagine each cell hypertrophic squirrel cage. Lot of turns (in a drill, for example, about 40), the field strength is much higher. Due to a sharp rise at the start of the energy consumption would be too great. Level EMF significant (defined rate of magnetic flux change). the rotor circuit is supplemented by a rheostat, trying to compensate for the lack. Active resistance reduces current, naturally a response reducing field generated by the conductors.

A rotor phase can improve characteristics of asynchronous motors, two or three conductors (roughly) provide greater traction. By cons technical solutions include the presence of current collectors, a brush unit. To reduce wear in some engines after induction rotor revolutions set shorted special mechanism. Much extended life of the equipment.

We see no reason to consider more slip-ring motors, will serve as the best illustration of reinforced squirrel cage. Imagine: instead of one was forty! Quantity (40 and down) is governed by the resistance of the rheostat.

How to set the speed of the induction motor

Anyone, including a three-phase asynchronous electric motor is unable to develop a speed close to the frequency of the field. Number of poles tend to be lowered. But even in this case, rarely achieve the desired 3000 rev / min (50 Hz x 60 s). In principle, it is impossible. Increasing the number of stator poles is practiced to reduce the speed as described above in Example outdoor fan.

The most common practice to connect the induction motor with squirrel-cage rotor in the phase amplitude control. The technique makes it as easy to get results. The currents of induction motors are large at the start, "due to" loss of the rotor core (reduced with increasing revolutions). We can not say that repair their own hands stator belongs to the category of simple, but much better than the wind rotor collector. Simple construction industry is due to the love to this kind of devices.